I don't know also why you say you couldn't rbf? Surely they'd just accept the new transaction on the other end?
They use a payment processor and don't accept BTC directly and it says specifically to send only one transaction to that address and that too within a time frame and if you send more than that it might not get credited because the conversion price might change in the meantime.
There's no model to predict the fee market and that's the best estimate that can be given. If the algorithm were to be more forgiving, people would complain that they're paying more than required for most of their transactions.
That's a problem then. Especially when you need a tx to get confirmed quickly. I usually do end up sending with higher using same ETA and block explorer telling that I paid like 400% higher fee.
Your best bet is to check the current status of the mempool yourself before sending coins to determine what should work for a quick confirmation.
Go to
https://jochen-hoenicke.de/queue/#1,2h and pay attention to the last graph on the site. Place your cursor all the way to the far right to see the fee ranges of all currently unconfirmed transactions that are waiting to be picked up. For a quick confirmation in the next few blocks, I would say you need 50-60 sat/vbyte right now. Unless there is a sudden spike of transactions with much higher fees, that should be more than enough.
Personally, I would never pay such fees, unless it is absolutely necessary.
Yes, I was lazy to do it. I should have checked such websites which tell the best current fee.
I suggest you use the "In the next block" option next time, again it's not a guarantee but still your transaction stands a higher chance of getting confirmed quicker
I prefer the Mempool fees estimation option along side a couple of tools like mempool.space. They give a better picture of the mempool in real time unlike ETA which is mostly based on the previous block
I do generally use that option but it was sending with way higher fee in the last couple of transactions I did. Like fee up-to 4 USD for 100 USD tx and I moved the scroller to the left and it said within 2 blocks which wasn't too bad, so I sent it with that option and it got stuck.
I'll try the mempool option next time. I couldn't understand what it meant is why I didn't give it a try. So 0.1 MB from tip means it will get picked up as soon as possible? like in next block and 0.2 within 2 blocks?